1. Field of the Invention
The present invention relates to a variable valve timing control apparatus for an internal combustion engine
2. Description of Related Art
Recently, the more and more internal combustion engines mounted on the vehicles are provided with hydraulic variable valve timing units that change valve timing of opening and closing an intake valve or an exhaust valve of the engine in order to increase the output, to improve the fuel efficiency, and to reduce exhaust gas emission. As shown in JP-A-2007-224744 and JP-A-2004-251254, a feed-back control amount and a hold control amount (hold duty) are used in the computation of a control amount (VCT control amount, or control duty) of a hydraulic control valve, which controls oil pressure actuating the variable valve timing unit. For example, the feed-back control amount is based on a difference between target valve timing (target VCT phase) and actual valve timing (actual VCT phase), and the hold control amount indicates a duty required to maintain the actual valve timing. The hydraulic control valve is actuated based on the above control amount such that a flow amount (oil pressure) of hydraulic oil, which is supplied to advance chambers and retard chambers of the variable valve timing unit, is changed. As above, the valve timing is advanced or retarded.
In the above, in view of the manufacturing variation of the variable valve timing unit and the hydraulic control valve hold control amount and aging of the same, the hold control amount is learned in the conventional art. In the conventional learning process of learning the hold control amount, when the actual valve timing is stable generally at the target valve timing (or when the difference between the actual valve timing and the target valve timing remains equal to or less than a predetermined value), the control amount of the hydraulic control valve in the above state is learned as the hold control amount, and the learning value is stored in the memory for update.
Also, in the hydraulic variable valve timing unit, as described in JP-A-H9-324613 (corresponding to U.S. Pat. No. 5,738,056) and JP-A-2001-159330 (corresponding to U.S. Pat. No. 6,330,870), a lock phase during the stopping of the engine is set at a middle of an adjustable range of the VCT phase such that the adjustable range of the valve timing (VCT phase) is enlarged. In the above, the intermediate lock position, at which the phase is locked during the stopping of the engine, is set at a phase suitable for starting the engine. Thus, the engine is started while the phase is at the intermediate lock position. Then, when oil pressure is increased to an appropriate oil pressure due to the engine rotation increase (oil pump rotation increase) after the completion of the engine start, the lock is released in order to start the feed-back control of the valve timing.
In the variable valve timing unit having an intermediate lock mechanism as in JP-A-H9-324613 and JP-A-2001-159330, a control characteristic of the VCT phase has multiple different control ranges. In general, if the control characteristic of the VCT phase varies with each of the control ranges, the hold control amount varies with each of the control ranges.
Thus, it has been proposed that the hold control amount is learned for each control range, and that the VCT control amount is computed by using a learning value of the hold control amount in the control range, within which the target VCT phase stays. However, because a boundary of the control ranges varies with manufacturing variation and aging of the variable valve timing unit, it is difficult to determine the control range, within which control range the target VCT phase of interest stays, when the target VCT phase is positioned around the boundary.
In JP-A-2002-295276, when a learning correction amount of the hold control amount becomes equal to or greater than a predetermined value, the boundary of the control ranges is learned. However, it takes substantially long time to learn the boundary, and also the likelihood of the erroneous learning becomes higher. Thus, the deterioration of the accuracy in the control around the boundary may be unavoidable disadvantageously.